1. Field of the Invention
The present invention relates to an x-ray image processing device for diagnostic use and industrial use.
2. Description of the Prior Arts
An X-ray image or X-ray picture of an object can be obtained by the difference of the X ray amounts passed every part of the object or as the contrast of the X ray amounts of the every part of the object, wherein the difference of the X-ray amounts are caused by the difference of the thickness and/or difference of the composition of the object. For example, in human tissues. The composition of bone is mainly calcium, and since the X-ray absorption amount of the bone is greater than other tissue, there can be displayed an image having contrast between the bone and other tissue.
In order to obtain an X-ray image of only bone or only other soft tissue, there has been known an energy subtraction method or a dual energy projection radiography wherein two X-rays of different energy such as 80 KV (kilo volts) and 120 KV of the X-ray excitation voltage are independently radiated to the object and the respective X-ray amounts of the X-ray images of the different energies are subjected to logarithmic conversion and differentiation or subtraction between the logarithmically converted values.
A conventional energy subtraction method will be explained hereinafter with reference to FIG. 1. A low energy X-ray image 1 and a high energy X-ray image 2 are applied to subtraction processing units 3 and 4 respectively. The low energy X-ray image 1 is a logarithmic conversion signal of a X-ray image information taken by using a low energy X-ray obtained a X-ray tube excited by such as 80 KV and the high energy X-ray image 2 is a logarithmic conversion signal of a X-ray image information taken by using a high energy X-ray obtained by a X-ray tube excited by such as 120 KV. Both of the X-ray images 1 and 2 are subjected to the subtraction processing in the subtraction processing 3 and 4 so as to obtain the processed image 5 and the processed image 6. The equations used in the respective subtraction processing 3 and 4 are EQU image 5=k1.times.image 1-k2.times.image 2 (1) EQU image 6=k3.times.image 1-k4.times.image 2 (2)
In general, the picture quality of the image obtained by using the above equations is much deteriorated.
Assuming that the noise components of the images 1 and 2 are N1 and N2, the noise components of the processed images 5 and 6 are shown by EQU (noise of image 5).sup.2 =(K1.times.N1).sup.2 +(K2.times.N2).sup.2( 3) EQU (noise of image 6).sup.2 =(K3.times.N1).sup.2 +(K4.times.N2).sup.2( 4).
As shown by the equations (3) and (4), the noise component of the obtained image is the sum of the square values of the noise of the original images. Particularly, in case of taking out a picture component from the images of which contrast difference is small or in case of taking out a image component of which a space frequency is high, the picture quality is much deteriorated. In addition, since, in the high energy X-ray, the difference of the absorption due to the difference of the composition is small compared to the low energy X-ray, the X-ray image of the high energy X-ray has a low contrast difference, the signal to the noise ratio is bad and therefore, the signal to noise ratio is much deteriorated.